One alcohol of great interest today is ethanol. Most of the fuel ethanol in the United States is produced from a wet mill process or a dry grind ethanol process. Although virtually any type and quality of grain can be used to produce ethanol, the feedstock for these processes is typically corn.
The conventional processes for producing various types of alcohol from grain generally follow similar procedures. Wet mill corn processing plants convert corn grain into several different co-products, such as germ (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal feed), and starch-based products such as ethanol, high fructose corn syrup, or food and industrial starch. Dry grind ethanol plants generally convert corn into two products, namely ethanol and distiller's grains with solubles. If sold as wet animal feed, distiller's wet grains with solubles are referred to as DWGS. If dried for animal feed, distiller's dried grains with solubles are referred to as DDGS. In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kg (approximately 17 lbs.) of DDGS in addition to the approximately 10.5 liters (approximately 2.8 gal) of ethanol. This co-product provides a critical secondary revenue stream that offsets a portion of the overall ethanol production cost.
With respect to the dry grind process, FIG. 1 is a flow diagram of a typical dry grind ethanol production process 10. As a general reference point, the dry grind ethanol process 10 can be divided into a front end and a back end. The part of the process 10 that occurs prior to distillation and dehydration 24 is considered the “front end”, and the part of the process 10 that occurs after distillation and dehydration 24 (hereinafter “dehydration”) is considered the “back end”. To that end, the front end of the process 10 begins with a grinding step 12 in which dried whole corn kernels are passed through hammer mills for grinding into meal or a fine powder. The screen openings in the hammer mills typically are of a size 7/64, or about 2.78 mm, with the resulting particle distribution yielding a very wide spread, bell type curve, which includes particle sizes as small as 45 micron and as large as 2 to 3 mm.
The grinding step 12 is followed by a liquefaction step 16 whereat the ground meal is mixed with cook water to create a slurry and a commercial enzyme called alpha-amylase is typically added (not shown). The pH is adjusted here to about 5.8 to 6 and the temperature maintained between about 50° C. to 105° C. so as to convert the insoluble starch in the slurry to soluble starch. Various typical liquefaction processes, which occur at this liquefaction step 16, are discussed in more detail further below. The stream after the liquefaction step 16 has about 30% dry solids (DS) content with all the components contained in the corn kernels, including sugars, protein, fiber, starch, germ, grit, and oil and salts, for example. There generally are three types of solids in the liquefaction stream: fiber, germ, and grit, with all three solids having about the same particle size distribution.
The liquefaction step 16 is followed by a simultaneous saccharification and fermentation step 18. This simultaneous step is referred to in the industry as “Simultaneous Saccharification and Fermentation” (SSF). In some commercial dry grind ethanol processes, saccharification and fermentation occur separately (not shown). Both individual saccharification and SSF can take as long as about 50 to 60 hours. Fermentation converts the sugar to alcohol using a fermentor. Subsequent to the saccharification and fermentation step 18 is the distillation (and dehydration) step 24, which utilizes a still to recover the alcohol.
Finally, the back end of the process 10, which follows distillation 24, includes a centrifugation step 26, which involves centrifuging the residuals, i.e., “whole stillage”, produced with the distillation step 24 to separate the insoluble solids (“wet cake”) from the liquid (“thin stillage”). The “wet cake” includes fiber, of which there are three types: (1) pericarp, with average particle sizes typically about 1 mm to 3 mm; (2) tricap, with average particle sizes about 500 micron; (3) and fine fiber, with average particle sizes of about 250 micron. The liquid from the centrifuge contains about 6% to 8% DS.
The thin stillage enters evaporators in an evaporation step 28 to boil away moisture, leaving a thick syrup that contains the soluble (dissolved) solids from fermentation (25% to 40% dry solids). The concentrated slurry may be subjected to an optional oil recovery step 29 whereat the slurry can be centrifuged to separate oil from the syrup. The oil can be sold as a separate high value product. The oil yield is normally about 0.4 lb./bu of corn with high free fatty acids content. This oil yield recovers only about ¼ of the oil in the corn. About one-half of the oil inside the corn kernel remains inside the germ after the distillation step 24, which cannot be separated in the typical dry grind process using centrifuges. The free fatty acids content, which is created when the oil is held in the fermenter for approximately 50 hours, reduces the value of the oil. The (de-oil) centrifuge only removes less than 50% because the protein and oil make an emulsion, which cannot be satisfactorily separated.
The centrifuged wet cake and the syrup, which has more than 10% oil, can be mixed and the mixture may be sold to beef and dairy feedlots as Distillers Wet Grain with Soluble (DWGS). Alternatively, the syrup can be mixed with the wet cake, then the concentrated syrup mixture may be dried in a drying step 30 and sold as Distillers Dried Grain with Soluble (DDGS) to dairy and beef feedlots. This DDGS has all the protein and 75% of the oil in corn. But the value of DDGS is low due to the high percentage of fiber, and in some cases the oil is a hindrance to animal digestion.
Further with respect to the liquefaction step 16, FIG. 2 is a flow diagram of various typical liquefaction processes that define the liquefaction step 16 in the dry grind ethanol production process 10. Again, the front end of the process 10 begins with a grinding step 12 in which dried whole corn kernels are passed through hammer mills for grinding into meal or a fine powder. The grinding step 12 is followed by the liquefaction step 16, which itself includes multiple steps as is discussed next.
Each of the various liquefaction processes generally begins with the ground meal being mixed with cook, or back set, water, which can be sent from evaporation step 28 (FIG. 1), to create a slurry at slurry tank 32 whereat a commercial enzyme called alpha-amylase is typically added (not shown). The pH is adjusted here, as is known in the art, to about 5.8 to 6 and the temperature maintained between about 50° C. to 105° C. so as to allow for the enzyme activity to begin converting the insoluble starch in the slurry to soluble starch.
After the slurry tank 32, there are normally three optional pre-holding tank steps, identified in FIG. 2 as systems A, B, and C, which may be selected depending generally upon the desired temperature and holding time of the slurry. With system A, the slurry from the slurry tank 32 is subjected to a jet cooking step 34 whereat the slurry is fed to a jet cooker, heated to 120° C., held in a U-tube for about 5 to 30 min., then forwarded to a flash tank. The jet cooker creates a sheering force that ruptures the starch granules to aid the enzyme in reacting with the starch inside the granule. With system B, the slurry is subjected to a secondary slurry tank step 36 whereat steam is injected directly to the secondary slurry tank and the slurry is maintained at a temperature from about 90° C. to 100° C. for about 30 min to one hour. With system C, the slurry from the slurry tank 32 is subjected to a secondary slurry tank-no steam step 38, whereat the slurry from the slurry tank 32 is sent to a secondary slurry tank, without any steam injection, and maintained at a temperature of about 80° C. to 90° C. for 1 to 2 hours. Thereafter, the slurry from each of systems A, B, and C is forwarded, in series, to first and second holding tanks 40 and 42 for a total holding time of about 2 to 4 hours at temperatures of about 80° C. to 90° C. to complete the liquefaction step 16, which then is followed by the saccharification and fermentation step 18, along with the remainder of the process 10 of FIG. 1. While two holding tanks are shown here, it should be understood that one holding tank or more than two holding tanks may be utilized.
To increase the alcohol yield, and generate additional revenue, for example, from oil and/or protein yields in the typical dry mill process, it would be beneficial to develop a process(es) to further break-up the initially ground germ particles and grit particles, which include mostly starch, to release more starch, oil, and/or protein therefrom. Such a process could provide for increased alcohol, oil, and/or protein yield, and produce much higher purity fiber (with less protein, starch and oil), which can be used as a raw feed stock for the paper industry and cellulosic to secondary alcohol processes.
Various dry grind systems have attempted to increase alcohol yields, for example, by focusing on the grinding aspect in the dry grind process 10. However, such systems are known not to have produced very good results. For example, with the grind systems in today's market, these systems tend to decrease the size on all of the particles (fiber, germ, and grit) at the same time and at the same rate. The resulting corn components can be difficult to separate, particularly if all of the particles, including the fiber, are ground too small, e.g., less than 300 microns. While alcohol yield may improve with smaller particle sizes, this can also produce a very wet decanter cake and dirty overflow, i.e., dirty thin stillage. And this dirty overflow can create fouling and result in lower syrup concentrations during the evaporation step 28. Lower syrup concentrations and wetter cakes also produce increased dryer loads raising the drying costs of DDGS. In contrast, if the resulting corn components are too large in size, e.g., greater than 1000 microns, the particles will not adequately convert to sugar during the liquefaction step 16 and alcohol yield, for example, will drop.
Such conventional systems also tend to focus on either grinding the entire stream or a partially separated stream in a very wet slurry form, without any dewatering prior to grinding. For grinding solid particles, the feed that is sent to the grind mill should be as dry as possible to yield maximum grinding results. Current systems also have failed to remove fine solid particle before feeding the particles to the cutting/grinding device. As such, the fine solid particles become smaller particles, i.e., too small, creating problems on the back end of the process by producing very wet cakes and dirty overflow, as discussed above.
It would thus be beneficial to provide an improved milling method(s) and system(s) for dry grind ethanol plants that can improve alcohol, oil, and/or protein yields, and generate additional revenue from oil and/or protein yields, for example, while avoiding and/or overcoming the aforementioned drawbacks.